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learn-java8 Documentation

repository·master·Indexed 23 days ago

https://github.com/hellokaton/learn-java8

Educational materials and source code for a Java 8 course focusing on functional programming and modern development. Covers asynchronous programming with CompletableFuture, concurrency utilities including ExecutorService, ScheduledExecutorService, and atomic operations with AtomicInteger, as well as locking mechanisms such as ReentrantLock, ReentrantReadWriteLock, and StampedLock.

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What's inside learn-java8

  1. Overview of the Learn Java 8 Course

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    The learn-java8 repository contains the source code for the 'Learn Java 8' video course. The course focuses on core Java 8 features, their use cases, and best practices for functional programming.

    Each lesson follows a What, Why, and How teaching methodology:

    1. What: Defining the skill/feature.
    2. Why: Explaining the rationale behind it.
    3. How: Demonstrating usage through code examples and real-world scenarios.

    The course concludes with discussions on Java 8 best practices and correct functional programming patterns.

  2. What is a Java 8 Stream and how does it work?

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    A Stream is a high-level abstraction introduced in Java 8 for processing collections of data. Unlike traditional collections, a Stream does not store data; instead, it acts as a pipeline through which data from a source flows.

    Key Characteristics:

    • Single-use: A stream can only be traversed once. Once elements are "consumed" at the end of the pipeline, you must create a new stream from the original data source to process them again.
    • Internal Iteration: Instead of writing manual loops (external iteration), you provide instructions (declarative style) to the stream, and the Java runtime handles the iteration process internally. This allows the JVM to optimize processing, including multi-threading.

    The Stream Pipeline Process:

    1. Prepare a Data Source: Create a stream from a collection, array, value, or file.
    2. Intermediate Operations: Apply one or more transformations (e.g., filtering, mapping) that return a new stream, allowing for method chaining.
    3. Terminal Operation: Execute a final operation that triggers the processing and returns a result (e.g., a value, an Optional, or a collection).

  3. Why use CompletableFuture instead of Future

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    Standard Future.get() methods are blocking, which can reduce system responsiveness. While Future.get(long, TimeUnit) allows for timeouts, it doesn't solve the underlying issue of thread blocking.

    CompletableFuture provides a non-blocking way to handle asynchronous results. For example, you can use thenAccept to trigger a callback (like send(Response)) asynchronously once a task completes. This allows the main execution thread to finish quickly without waiting for the background task, improving overall system throughput and maintainability.

  4. Use Lambda Expressions and Functional Interfaces

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    Java 8 introduces Lambda expressions (closures) to allow passing functions as parameters or treating code as data. A Lambda expression consists of a comma-separated parameter list, the -> symbol, and a statement block.

    To ensure compatibility, Java 8 uses Functional Interfaces—interfaces that contain exactly one abstract method. Examples include java.lang.Runnable and java.util.concurrent.Callable.

    To prevent accidental breakage (e.g., adding a second method to an interface), use the @FunctionalInterface annotation to explicitly declare an interface as functional.

  5. Perform multi-level grouping with groupingBy

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    Data grouping (similar to SQL GROUP BY) can be achieved using Collectors.groupingBy. To perform multi-level grouping (grouping elements within groups), use the overloaded groupingBy method that accepts two parameters:

    1. First parameter: The condition for the primary (first-level) grouping.
    2. Second parameter: A new groupingBy function containing the condition for the secondary (second-level) grouping.

  8. Work with non-ISO Calendars

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    While Java uses the ISO 8601 calendar system by default, it supports several other chronologies via the ChronoLocalDate interface:

    • ThaiBuddhistDate (Thai Buddhist calendar)
    • MinguoDate (Republic of China calendar)
    • JapaneseDate (Japanese calendar)
    • HijrahDate (Islamic calendar)

    Conversion and Usage

    You can convert a standard LocalDate to a specific calendar using the .from() method. For generic handling of different locales, use Chronology.

    Best Practice: Avoid using ChronoLocalDate for core business logic. Use LocalDate for storage and calculations to avoid errors caused by differing calendar rules (like leap months or era changes). Only use ChronoLocalDate when you need to localize input or output.

    // Convert to Japanese Date
LocalDate date = LocalDate.now();
JapaneseDate jpDate = JapaneseDate.from(date);

// Generic handling via Chronology
Chronology jpChronology = Chronology.ofLocale(Locale.JAPANESE);
ChronoLocalDate jpChronoLocalDate = jpChronology.dateNow();

  9. Use StampedLock for high-throughput concurrency control

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    Introduced in Java 8 (java.util.concurrent.locks), StampedLock provides three modes of access to control reading and writing. It is designed to address the starvation issues found in ReentrantReadWriteLock when there are many readers and few writers.

    Lock Modes

    1. Write Lock: Provides exclusive access. writeLock() may block the current thread until access is granted, returning a stamp used for releasing the lock via unlockWrite(stamp). tryWriteLock() is also available for non-blocking attempts.
    2. Read Lock (Pessimistic): Provides non-exclusive access. readLock() may block the thread, returning a stamp used for releasing via unlockRead(stamp). tryReadLock() is also available.
    3. Optimistic Read: A weak version of a read lock that does not block writers. tryOptimisticRead() returns a non-zero stamp only if no write lock is currently held. After performing the read, you must call validate(stamp) to check if a write occurred during the operation. This mode is ideal for short-duration reads to reduce contention and increase throughput.

    Key Concepts

    • Pessimistic Lock: Assumes conflicts will occur and locks the data immediately, blocking others until the operation is complete (e.g., synchronized).
    • Optimistic Lock: Assumes conflicts are unlikely. It performs operations without locking and only checks for data integrity (via validation) at the time of the update/commit.

  10. Use Default and Static Methods in Interfaces

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    Java 8 extends interfaces with two new types of methods:

    1. Default Methods: These allow adding new functionality to existing interfaces without breaking binary compatibility. Unlike abstract methods, default methods have a body and do not require implementing classes to override them. They are inherited by implementing classes. Examples include new methods in java.util.Collection like stream(), parallelStream(), forEach(), and removeIf().
    2. Static Methods: These allow defining utility methods directly within an interface.

    Caution: Use default methods carefully in complex inheritance hierarchies to avoid ambiguity and compilation errors.

  11. Partition data using partitioningBy

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    Partitioning is a special case of grouping where the classification function returns a Boolean. This results in a Map<Boolean, List<T>> containing at most two groups: one for true and one for false. This is useful when you want to keep both sets of elements (those that satisfy a predicate and those that do not) in a single operation.